Plant material wastes (biomass) are made up of five main components: cellulose, hemicellulose, lignin, crude protein and ash. Cellulose is generally a linear, unbranched glucose-based homopolymer, i.e., a polysaccharide, of relatively high molecular weight. Hemicellulose is typically a branched and/or unbranched polymer of D-glucose, D-mannose, L-arabinose and D-xylose of about 100-200 sugar residues per polymer chain. Lignins are amorphous crosslinked phenolic polymers that occur uniquely in vascular plants and comprise 20-30% of most wood.
Processing of biomass is important in several industries such as waste management, pulp and paper, food manufacture, and energy production among others. For example, it is known to hydrolyze cellulosic materials into monosaccharides for varying purposes including feed stocks for other chemicals, food stuffs, fuels, and the like. In addition, conversion of biomass to sugars usable directly as food or as chemical reagents is an interest in planning long-term space missions. In space, the fixation of carbon dioxide into edible biomass can be performed by agronomical species such as wheat, potatoes, soybeans, and the like. However, only half of the crop is edible. Of the inedible portion, approximately 50-68 percent is polysaccharide which can be reduced into fermentable sugars. The remainder is primarily unusable lignin. Waste paper produced in space is another source of usable polysaccharide since it is primarily cellulose.
Reduction of polysaccharides by hydrolysis is well known in the art. Two basic methods are generally used: (1) chemical treatment, e.g., reduction using an acid catalyst; and (2) biological breakdown using enzymes or microbes such as fungus. Such methods generally include one or more pretreatments to increase hydrolysis reaction rate and yield. Pretreatments typically increase the availability and surface area of reducible polysaccharides by disturbing the physical and molecular structure of the feed material and/or fractionating the lignocellulosic material into its lignin, hemicellulose and cellulose components.
Examples of common pretreatment techniques include milling and sizing; steam solubilizing in the presence of chemicals such as caustic widely used in pulp and paper manufacture, ammonia, chlorite, sulfur dioxide, amines, acids both dilute and concentrated, etc.; autohydrolysis by high temperature steam (i.e., 220.degree.-275.degree. C.); steam explosion (a forceful extrusion of a steam/feed mixture through an orifice by a sharp pressure drop across the orifice); and electron irradiation. In a given process, common pretreatments may be combined. Additional details surveying the prior pretreatment art are described in Petersen et al., The Engineering Society for Advancing Mobility Land Sea Air and Space (SAE International) technical paper 901282, Jul. 9-12, 1990.
The drawbacks of both of these types of processes are evident. The reaction rate of enzymatic hydrolysis is low and a significant concentration of undesirable byproducts such as furfural can result. Concentrated acid-catalyzed hydrolysis produces little or none of the byproducts seen in enzymatic hydrolysis and has a higher reaction rate, but acid consumption is high and product recovery from the reaction effluent is expensive. Health and environmental hazards are also present. Dilute acids can be used, but sugar degradation and yield reduction can, and usually do occur. Pretreatments in both these processes consume energy in the form of steam. Where chemicals are used, they must be removed from the end product. Even with the prior art pretreatment, enzymatic hydrolysis is relatively slow and undesired byproducts are still present.
In space missions, a unique set of constraints are presented. The expendable materials used in the process must either be carried as part of the mission payload or be produced on board the space habitat or vehicle. The availability of energy sources is likewise limited. While sulfuric acid is an excellent catalyst for the hydrolysis reaction, it is difficult to manufacture in space. Enzymes, on the other hand, can theoretically be made in space, but the primary problem of low yield remains. The constraints of space limit the suitability of pretreatments using steam and many chemicals since the preparation and recycle of the required chemicals is generally difficult. Steam production is also energy intensive and the furfural byproduct requires a relatively complex separation step.
J. Azuma et al., Journal of Fermentation Technology, Vol. 62, No. 4, pp 377-384 (1984), discloses a microwave radiation pretreatment method for enzymatic saccharification of lignocellulosic wastes. Enzymatic susceptibility of the wastes is said to be improved by aqueous microwave pretreatment above 160.degree. C. with maximum improvement at 223.degree.-228.degree. C. Following pretreatment, maximum yield of reducing sugars is 77-84% of the polysaccharide present in the original lignocellulose waste.
P. J. Blotkamp et al., American Institute of Chemical Engineering (AIChE) Symposium Series No. 181, Vol. 74(1981), describes a simultaneous saccharification of cellulose and fermentation to ethanol utilizing enzymes of the mold Trichoderma reesea and the yeast Saccharomyces cerevisiae. The hydrolysis rate was said to increase when compared to saccharification alone due to removal of competitive inhibition by glucose and cellobiose.
Accordingly, there remains a need for low energy consumption, low complexity apparatus and method for the conversion of cellulosic wastes into soluble saccharides suitable for terrestrial or space use wherein the chemicals utilized are not extremely hazardous, easily recyclable and can be prepared in space. Furthermore, it would be advantageous if such an apparatus and method produced soluble saccharides without unwanted decomposition byproducts such as furfural.